1. Field of the Invention
The present invention relates to a thin film transistor liquid crystal display(hereinafter "TFT-LCD"), and more particularly to a TFT-LCD having a vertical thin film transistor.
2. Description of the Related Art
Liquid crystal displays used in television and other graphic display devices have been developed for substituting the cathode ray tube("CRT"). Particularly, the TFT-LCD has an advantage of excellent response characteristic and is appropriate for high number of pixels. Therefore, high quality and large size of display devices comparable to the CRT, is realized.
The TFT-LCD comprises a TFT array substrate in which a TFT and a pixel electrode are formed, a color filter substrate in which a color filter and a counter electrode are formed, and a liquid crystal layer sandwiched between the TFT array substrate and the color filter substrate.
FIG. 1 is a plane view showing a TFT array substrate of a conventional TFT-LCD. As shown in the drawing, a plurality of gate lines 2 are arranged in rows and a plurality of data lines 8 are arranged perpendicular to the gate lines 2. A pixel electrode 7 made of transparent metal layer, for example an ITO metal layer, is disposed within a pixel area defined by a pair of gate lines 2 and a pair of data lines 8. A TFT 10 is disposed at a portion of intersection of the gate line 2 and the data line 8. The TFT 10 includes a gate electrode, i.e. a part of the gate line 2, a drain electrode 8b withdrawn from the data line 8, and a source electrode 8a opposed to the drain electrode 8b and in contact with the pixel electrode 7.
FIG. 2 is a cross-sectional view taken along the line II-II' of FIG. 1. As shown in the drawing, a transparent insulating substrate, for example a glass substrate 1 is provided and a gate electrode 2a is formed on the glass substrate 1. A gate insulating layer 3 is deposited on the glass substrate 1 to cover the gate electrode 2a, and an etch stopper 4 is formed on the gate insulating layer portion of an upper portion of the gate electrode 2a.
A semiconductor layer 5 and an ohmic contact layer 6 of a stacked structure are formed to extend at both sides of the etch stopper 4. The semiconductor layer 5 is made of an undoped amorphous silicon and the ohmic contact layer 6 is made of a doped amorphous silicon. A pixel electrode 7 is formed on the gate insulating layer portion corresponding to the pixel area. Source and drain electrodes 8a, 8b made of an opaque metal are formed on the ohmic contact layer 5, thereby forming a TFT 10. The source electrode 8a is formed in contact with the pixel electrode 7.
The TFT-LCD having the TFT array substrate of the foregoing structure has shortcomings as follows.
First of all, the semiconductor layer of the TFT is made of the undoped amorphous silicon as previously described. However, since the amorphous silicon has low mobility, it is suitable for applying in the small size TFT-LCD, and it is not suitable for applying in the large size and high quality TFT-LCD. Meanwhile, a technique to use polysilicon having relatively high mobility instead the amorphous silicon, has been under development. However, in order to utilize this technique, already established production line should be changed thereby requiring further cost for investing the equipment.
Next, the source and the drain electrodes of the TFT are formed within the pixel area. However, since the source and the drain electrodes made of an opaque metal, the aperture ratio is degraded.